RNA Modification and Processing

Hi in this video we're gonna be talking about M. RNA modification and processing. So after transcription RNA has to undergo through um a few various processing steps before translation can occur because the M. RNA when it's trans transcribed is not at all ready to be translated. It has to be perfected and sort of beautified up so that the protein that is made from translation is correct. And so there's a few different things that happen. So the first one is that it gets this thing called a five prime cap and this is a cap of a residue called a method guano seen molecule. And you don't necessarily need to know what this is. But the five prime cap, it's just a molecule that gets added on to it. And that cap protects the RNA for degradation. And it will be important for translation. And we'll talk about how it's important for translation in the translation videos. But the first thing that happens is the five prime cap. Super important. The second thing that happens is it gets a poly ventilation tail at the three prime end. So what this is it's around 100 and 50 to 200. It can be more than that. But generally it's around this many adami nucleotide. So it's just a a 200 times and at the very end of the transcript and what triggers this. So there's an enzyme that adds this on. Right. And so what triggers it is there's actually a poly ventilation signal at the end of the transcript and that's what it looks like A. A. U. A. A. And this is the signal that triggers the addition of the poly A tail after it. This poly A tail is also super important because it allows it to be imported out of the nucleus so that it can be translated. And so here if we have an example. So here's the coding sequence of the M. R. N. A. We have the cap here we have the poly A tail. And then we have these two regions that the U. T. R. Stands for. Untranslated regions. And we'll talk about these um and other videos but just know that here's the cap, here's the coding sequence and here's the poly A tail. Now the third thing that happens is called splicing. And splicing removes the non coding segments of the transcript called N. Tron, from the coding segments of the transcript called Exxon's a piece of hair. Well let's get that out of there. Okay so splicing. So there's the bronze and this is non coding and exxons. And pretty much when you have an M. RNA transcript it looks kind of like this where you have the coding sequences the exxons intertwined with the N. Tron and that's what the entire transcript looks like. And so spicing says okay we're gonna cut you out, we're gonna cut you out, we're gonna cut you out. Whoops cut this out and the two Exxon's will come together and form a single transcript that splicing. So the enzyme that's responsible for this is called the splices Ohm And the splices ohm is a bunch of different proteins and enzymes. And RNA is, it's not just one thing and it cuts out the environs and what we called pre M. RNA. Because it's not M. RNA yet, it's not processed yet to form M. RNA. So the spices own is made up of RNA is called small nuclear RNA. Is if you want to know the names that you one U two, U four U five U six. Those are the RNA. And it's also made up of proteins. And so we call the splices own. We give it a special fancy name called the small ribonucleic protein complex. Or you may see its SNR and P. And this is the combination of the small nuclear RNA and the proteins that make up the splices own. And so this splice zone where the small rival nuclear approaching complex comes together. And it has to recognize certain sequences in order to splice. So what it does is it recognizes a five prime splice site which is always the G. U nucleotide. It recognizes a three prime splice splice site which is always A G. Which you will see this is the G. U. A. G. Roll. And it recognizes a third sequence called the branch point and the branch point is just a single add any nucleotide. So it's a single eh around 18 to 40 nucleotides upstream of the three prime splice site. So you have G. You, you have the A. And you have the A. G. And these are three sequences that are found in every single uh splice site where the splice is always going to cut it out. So what happens actually is um there's a structure called malaria which I'm about to show you, but it's a small circular structure which is formed through the N. Tron exercising. So what happens you see here? So you have an Exxon N. Tron and Exxon. And you can see our sites here, You have the G. You the A. G. And the branch point. So what happens is first thing is there's a cut at the G. U. And this forms the circular structure called Hilaria. See how it's like kind of circle, It's folded back in on itself with the branch point. And eventually this will get cut as well. This one gets cut here. And so the larry, it goes off into Neverland essentially and gets degraded. And then you have this spliced M. R. N. A. Now so far we've been working with two Exxon's right and my images. But you can have hundreds of Exxon's. And all of them have to be spliced. And so all of them can be spliced together. So you can have all 100 splice, you know, one right after the other. Or you can have various combinations of splicing. So you can have, you know, say 97 where three of them at some point have been removed. And you have a new combination of Exxon's and that's called alternative splicing when the exon's are put together but not necessarily in order and not necessarily all of them. So alternative slicing is a big thing that gives us a lot of different types of genetic diversity. And then another thing that happens is our final thing, it's called RNA editing. It doesn't necessarily happen that often you carry out but it's very common in pro carry outs in some lower eukaryotic organisms. And this is another form of post transcription allow RNA processing which is what we call anything that happens to the RNA after it's been transcribed post transcription all. So what happens is RNA editing is literally just changing the RNA sequence. And there's a bunch of different ways it can do that right? It can substitute. So a nucleotide is exchanged for another nucleotide. So if it was a and now it's gonna be you that substitution allele you have insertion allow and that's a nucleotide added. You have the opposite volitional which is when it's deleted. And so all of these different things can take a sequence that you know may otherwise code something and change it in some way so that it's now a different sequence. It has some type of genetic diversity. Like I said this doesn't happen so often in advanced eukaryotic organisms but it does happen um in some plants and some bacteria very often. And so how it knows where to do this. It's not just it's not just random it's not just random changing of nucleotides. Actually there are N. A. S. Called Guide RNA. S. These RNA choose where the RNA editing will occur. So we have an example like this. So here's a RNA sequence which is here by prime 23 prime. The GUIDE RNA is on top of it making it look like what we normally see. And this GUIDE RNA is actually complementary to the M. RNA that we're looking at changing. And this brings in proteins that will come in and it looks here the lead or add or substitute or whatever it's going to do. Um a nucleotide in this M. RNA sequence. So that's super important to editing that RNA. So with that let's now move on.

Which of the following is NOT a method of mRNA modification?

• A. 5' cap
• B. 3' Poly-A tail
• C. Methylation
• D. Splicing

The spliceosome is made up of which of the following components?

• A. DNA and RNA
• B. RNA and Protein
• C. Only RNA
• D. Only Protein

Which of the following is not a sequence that the spliceosome recognizes?

• A. 5' GU
• B. 3' AG
• C. Branch Point
• D. Splice Point

After transcription the RNA sequence cannot be changed or modified before translation.

• A. True
• B. False